The resultant of two vectors of magnitudes 3 units and 4 units is 1 unit. What is the value of their dot product?

Let θ  be the angle between the two vectors. The resultant is given by R2=A2+B2+2ABcosθ Putting the values of R, A and B we get(1)2=(3)2+(4)2+2×3×4×cosθ Or cosθ=−1  or θ=1800 Now A.B=ABcosθ=3×4×cos1800=−12 Hence the correct choice is (a).